Information handling systems utilize high speed printers for rapidly generating printed information in a tangible form. High speed printers generally utilize xerographic or impact printing technologies. Impact printers are desirable for low cost and required where the option to print multipart forms is desired. The printing mechanism for impact printers generally transfer ink or other material from a print ribbon onto the paper to form images on one major surface of the paper.
Continuous-form paper is usually supplied from a box in which the paper is stacked in a fan-fold pattern. The paper may be single layer or may be multi-layer to provide multi-part forms. Continuous-form paper is perforated along lateral lines for dividing the continuous length into separable rectangular sheets or forms. Each of the separable sheets is rectangular and is typically 11.5 inches high by 14 and 7/8 inches wide. The paper is folded along the perforations in a zigzag manner reminiscent of oriental hand fans in which each lateral perforation is folded in the opposite direction from the preceding fold to form a stack.
Tractor drives engage a longitudinal row of holes along each edge of the paper for moving the paper longitudinally from the source box of paper, through the printing mechanism and downward toward a horizontal surface upon which it refolds into an output stack of printed, continuous forms. The tractor drives tend to distort the paper at the tractor holes in the edges of the paper so the refold stack is bowed upward at the edges. Typically, the paper length remains slightly folded along the lateral perforations after unstacking and printing and the descending paper length naturally tends to refold onto the stack at each lateral perforation in the same direction that it was originally folded.
Since the introduction of fanfold paper refolding, practitioners have faced the problem that occasionally the paper will fail to refold along the lateral perforations in the proper direction, eventually a jumble of output builds up resulting in tearing of the sprocket holes as the tractors drive the paper against the jumble resulting in jamming of paper movement through the tractor drives. It is known that the misfolding is related to the bowing of the stack due to the tractor damage and to the height of the paper discharge above the top of the stack and is also related to the intermittent characteristics of feeding of the paper through the printer.
The longitudinal movement of the paper through the printer is not continuous. Usually the movement is stopped as each line is printed on the sheets. Also, the paper tends to move quickly through blank lines and even more quickly through blank pages. For a very high speed paper tractor, the paper output is often accelerated so that descending paper bends as it falls into the stack and fails to properly refold onto the stack.
Also, the printer may not operate continuously. The output typically consists of separate reports which are transmitted to the printer as desired so that the printer is idle for minutes or even for hours between jobs. In addition, information handling systems tend to be idle for long periods due to schedules of working shifts, weekends and holidays. The paper in the printer may be idle with a lateral perforation in a straightened configuration so as to forget the original fold direction at the perforation; or the paper may be idle in a bent configuration and retain the bend so that it does not properly refold onto the output stack.
Once the stack is started in the proper location with the continuous length of paper refolding in the previous fan-fold directions, proper refolding tends to continue without any additional aid. However, occasionally the paper fails to refold in the desired direction which produces an unfolded jumble of printed output, and eventually applies forces to the paper moving through the tractor. The tractor tears out the sprocket holes in the paper causing the paper to stop moving through the printer.
In order to minimize data loss due to paper jamming practitioners have instituted jam detection schemes. For example, the rotation of a follower wheel in contact with the longitudinally moving length of paper can be compared with the movement of the tractor drives to detect jams.
U.S. Pat. No. 4,810,239 to Moss, incorporated herein by reference, discloses a misfolding paper detector. "When a jam causes a portion of the paper forms to be forced upwardly from the stack, that portion urges the interposing member upwardly also to block the projected beam and disable the printer." U.S. Pat. No. 4,030,720 discloses "a photoelectric switch . . . to detect the paper." U.S. Pat. Nos. 4,227,683 to Spangler et al. and 4,504,051 to Bittner et al. disclose optically sensing the height of a stack of fanfold paper to control an elevator which vertically positions the stack.
In unrelated arts optical means are used to sense jams. For example, U.S. Pat. No. 4,734,744 to Yamamoto which relates to copy machines, "the jam is detected if the [light] beam is not obstructed at regular intervals" and U.S. Pat. No. 4,716,286 to Taylor, which relates to money dispensing, discloses "sensors provided at spaced intervals along the common acceleration device determine if bills have reached the acceleration device . . . the adaptive technique compensates for changes in the sensor such as component aging and dust accumulation."